The concept of shell freezing is well known in the prior art. It has been common place for some time now to rotate a flask of heat-sensitive material in a freezing solution of dry ice and acetone or similar freezing solution. The rotation of the flask brings the liquid material into contact with the cold wall of the flask where it freezes in an evenly distributed layer about the flask walls. If the material were to be freeze dried, the flask containing the frozen material would then be subjected to vacuum where the solvent could be more easily and quickly extracted due to the even layered surface area of the sample.
This simple shell freezer has been improved to provide a series of horizontal rollers submerged in a bath, the rollers rotating the flask within the bath. However, as freeze drying flasks generally have very large top openings, only a limited amount of liquid could be frozen in this manner without coming into contact with the top closure seal area and leaking out, or at least, destroying the seal. Thus, if too large an amount of liquid was placed in the flask to be freeze dried, the glass could not be stoppered after the freeze drying process was complete. A flask lift device was introduced to obviate this problem comprising a loop of bead chain or other material placed about the neck of the flask to support the rotating flask at an acute angle in the bath, thereby allowing more material to be frozen at one time. This approach would not work for all types and configurations of flasks, however. Further, while the mechanical rollers rotating the flask could have various spacings to accommodate different flask sizes, serious limitations still existed in the number and variety of flask diameters which would be used. The thermal transfer liquid in use is commonly alcohol, which poses an obvious fire hazard when the bath is not in use or is being circulated while cooled down to operating temperatures. This danger in the use of such a bath increases as a result of both its large volume and the electric stirrers necessary to circulate the bath. The large volume of alcohol necessary in this type of bath also necessitates a long wait when cooling the bath to operating temperatures. A further disadvantage in such shell freezers lies in the roller surfaces, which tend to slip on the surface of the flask when in the alcohol bath, thereby rotating the flask in an uneven manner and resulting in an uneven distribution of frozen material within the flask.
Therefore, an object of the subject invention is an improved shell freezer which will quickly and efficiently freeze a given quantity of liquid evenly about the interior of a flask.
Another object of the subject invention is a shell freezer which will accommodate a wider variety of flask sizes than heretofore possible.
A further object of the subject invention is a shell freezer which has a tiltable bath to allow a greater quantity of liquid to be frozen.
A still further object of the subject invention is the use of a minimum amount of heat exchange liquid while retaining the ability to freeze large amounts of sample in a short time.